FIG. 1 illustrates a well-known impedance matching circuit used to terminate data transmission links. Impedance-matching termination circuit 100 is generally implemented on-die (i.e. as an integrated circuit on a chip rather than as discrete components on a motherboard). Termination circuit 100 is used in single-ended and differential receivers on transmission lines for technologies such as high speed transceiver logic (HSTL) and stub series terminated logic (SSTL). Termination circuit 100 comprises PMOS transistor PMA, NMOS transistor NMB, resistors RA, RB and inverter INV coupled to input pad PAD. A signal is received at input node IN through input pad PAD. Input node IN is coupled to, for example, an input terminal of a comparator in a receiver (not shown).
Coupled in series between source voltage VDD and ground GND are PMOS transistor PMA, resistor RA, resistor RB and NMOS transistor NMB. Input pad PAD is coupled to the center of this series of components at input node IN between resistors RA and RB. Transistors NMB and PMA are controlled by an enable signal ENABLE. Because of the presence of inverter INV, when enable signal ENABLE is logic high both transistors NMB and PMA are turned on. When enable signal ENABLE is logic low, both transistors NMB and PMA are turned off. When transistors NMB, PMA are enabled, DC current IDC flows from VDD to ground GND through PMOS transistor PMA, resistor RA, resistor RB and NMOS transistor NMB.
The on resistance of PMOS transistor PMA and resistor RA together form a pull-up resistance RPULLUP. Likewise, the on resistance of NMOS transistor NMA and resistor RB together form a pull-down resistance RPULLDOWN. To obtain a common mode voltage of ½ VDD at input node IN, RPULLUP is designed to be equivalent to RPULLDOWN. The value of RPULLUP and RPULLDOWN depends on the impedance of the transmission line (not shown) coupled to input pad PAD. To avoid reflections, the output impedance of circuit 100, which is referred to as RTT, should match the transmission line impedance. In the Thevenin type termination shown in FIG. 1, the parallel combination of RPULLUP and RPULLDOWN must match the transmission line impedance. As such, the value of each of RPULLUP and RPULLDOWN should be twice the value of the transmission line impedance.
For example, if the transmission line impedance is 50 Ohms, then RPULLUP and RPULLDOWN will each be 100 Ohms (i.e. 2*RTT where RTT=50 Ohms) so that the output impedance of termination circuit 100 (i.e. RTT) will be 50 Ohms. In this case, DC current IDC would flow through 200 Ohms. Where VDD equals 1.8 Volts, as in the case of SSTL technology, then IDC would be 0.009 Amperes or 9 mA. A DC current of 9 mA is considerable and problematic considering chips, subject to limitations on power consumption, may have many inputs with on-die terminations. Further, wasteful power consumption should be eliminated where possible. Thus, at least to reduce power consumption, there is a need for improved termination circuitry for data transmission links.